I am sure you are researching as I post but here is my 2 cents. lol

Make the pond at least 3' deep, even if only the middle of you pond is that deep.

Level the edges, and research how you want the edges. I like the invisible liner look. A coping shelf 6" wide x 4" deep that has coping stones, with larger flat edge stones over, and overhanging about 2" over the pond edge. Also make a raised berm 2" high by at least 18" wide to keep run off out of the pond and to fit your edging rock.

Use the excavated soil to build up the waterfall area.

Install a bottom drain, or at least a retro-fit bottom drain. Greg Bickal sells a CD on the net with instructions. This will improve pond circulation. Savio and Pondsweep sell skimmers with retro-fit BD kits.

Did I mention to get a skimmer? lol

Research how to make a skippy filter or buy an out of pond filter. You will be glad you did- trust me.

Koi are beautiful- and demanding. They will eat your plants and they need very good filtration and oxygenation. I keep only 3 Koi in a 3500 gallon pond with a skimmer, UV, pressurized filter, and bio-waterfall, with 10% weekly water changes. I am upgrading this year. I don't think I knew what I was getting myself in to when I got those cute 6" Koi. They are now over 28" and I can not keep floating plants in the pond, and they tear up everything. Those crazy kids!

Oxygenating plants release oxygen during the day and absorb it at night. Plan on running your pump 24/7.

I suggest you do not put Koi in your pond due to size. I am not trying to be a Koi snob or anything- but I don't think they will do well when they get big, and they get big relatively fast. I love shubunkins for their personality and beauty. They would do well in a pond the size you are planning.

Big is not always better IMHO. My pond is very big for 150'x50' lot. I had to change the landscaping around to make it look natural. Expensive.

Landscaping- I suggest you incorporate evergreens around the pond edges such as spreading junipers, ect if you are using a lot of rocks. Ponds can look kind of dismal in February before things get growing.

Hope this helps. Good luck in planning and building your pond. Post pics when you get started and let us know if you run in to problems during construction.